The invention relates to a gear motor with an electric motor whose rotor has a hollow shaft which is disposed on an output shaft, and whose torque can be transferred to the output shaft through a gearing with at least one drive shaft and a clutch.
Gear motors in which the hollow shaft is permanently connected to the gearing or directly to the load are known. It is also known to configure the gearing as a shift gearing in order to be able to drive the load at different rotatory speeds. Such gear motors are often used when the load has to be driven at different speeds, as for example in a so-called "fast forward" mode or with a considerable reduction to enable the performance of slow operations. One especially interesting application is, for example, so-called crystal pulling apparatus in which crystals of semiconductor material are drawn by the Czochralski method from the molten material. For the slow-running pulling process a very low rotatory speed is required at the output end. To raise the finished crystal or lower a seed crystal holder to start another drawing process, however, a fast speed is needed as a rule so as to reduce the idle time of the pulling apparatus. For this purpose special shifting gear motors have been used in the past, which did not form a single unit with the electric motor. Also, the known gear transmissions lacked the great smoothness of operation required for a crystal pulling process, and lastly the known drives had an unfavorable ratio of torque to the size of their structure.
U.S. Pat. No. 4,660,669 discloses a drive of the class described above, which is called an electrical servo unit. In this case, however, the electric motor is provided only as an auxiliary drive, and the actual torque is applied to a drive shaft by an external drive unit. The auxiliary motor with its hollow shaft can be applied when needed, through gears and a magnetic clutch, to an output shaft. The drive shaft and output shaft are connected to one another resiliently through a torsion bar and leaf spring, but with the same speed, the torque angle being used to actuate a switch by which the auxiliary motor can be turned on and off. The gear transmission and clutch, therefore, do not serve to vary the rotatory speed ratio between the drive shaft and driven shaft, both of which are configured as solid shafts.
French Patent 1 437 856 discloses a gear motor in which the solid shaft of an electric motor can be applied to the output shaft through a single double-action clutch with two friction pairs either directly or via a planetary gear drive. Only the housing of the planetary gear drive has a hollow shaft. To avoid having to shift the motor shaft to operate the clutch, one of the two parts of the clutch is mounted for axial displacement. Because of the double-action clutch, this shifting takes place against the action of strong compression springs, so that a very strong pot magnet is provided to actuate the clutch. On account of the series arrangement of motor shaft, clutch, planetary drive and output shaft, this known system has a very great structural length and, due to the masses that have to be moved, the engagement of the clutch causes severe vibration of the unit.